The present invention relates in general to improved fusing apparatus and methods, and more particularly, to fusing apparatus and methods for fusing magnet wire having an electrically insulating coating to a terminal using a pair of opposed electrodes having concave shaped end faces.
In the process commonly referred to as "fusing", a pair of opposed electrodes are applied to the elements of an electrically conductive workpiece, and electrical current is directed through the electrodes so as to form a bond between the workpiece elements. In this arrangement, one electrode referred to as to the "fusing" electrode engages an element of the workpiece adjacent the juncture between elements, and substantial forces are applied through this electrode so as to force the workpiece elements together. The opposite electrode, normally referred to as the "ground" electrode engages one or another of the workpiece elements remote from the juncture. The fusing electrode typically has a relatively high resistivity. An electrical potential is applied across the electrodes causing current to flow through the electrodes and the workpiece. Substantial heat is generated in the resistive fusing electrode so that heat is transferred from the fusing electrode to the engaged workpiece element. The workpiece elements fuse with one another under the combined influences of heat transferred from the fusing electrode and pressure applied by the fusing electrode. By contrast, in the bonding process referred to as spot welding, the major portion of the heat evolved during the process is generated by the electrical resistance within the workpiece itself. Both processes, however, involve application of electrical current by means of electrodes engaged with the workpiece.
Considerable difficulties have been encountered heretofore in making electrical circuit assemblies which include relatively fine wire leads which have a coating of plastic electrical insulation, commonly referred to as magnet wire. In manufacture of an electronic assembly, an electrical connection must be established between the ends of the magnet wire and metallic conductors, for example, terminal pins.
Fusing the wire magnet to the terminal pins is, however, difficult because the electrical insulation on the wire prevents electrodes used in the fusing operation from making a closed circuit. Thus, in order to fuse the magnet wire to the terminal pins, the insulation must first be stripped off of the magnet wire. Such a stripping operation is tedious and time consuming. Also, because the magnet wire is directly forcibly contacted by the electrodes, the wire size and accordingly its mechanical strength can be impaired to an extent sufficient to cause the wire to break, thereby completely destroying the current carrying capability of the wire.
There is known from Riordan et al., U.S. Pat. No. 4,687,898 a method of fusing a magnet wire to a terminal pin of an electronic assembly such as a coil bobbin which overcomes the problems and disadvantages discussed hereinabove. The terminal pin of the coil bobbin has opposed sets of notches sized, shaped and spaced so as to permit the magnet wire to be wrapped around the terminal pin in a spiral path defined by the notches. Because the magnet wire is received in the notches, the notches function to properly locate the wire on the pin and to maintain the wire stationary. The notches have a depth selected so that the portions of the magnet wire received in the notches are recessed below an outer surface of the pin, whereby electrodes used to fuse the magnet wire to the pin are allowed to contact the pin and thereby form a closed circuit. The resulting heat melts the insulation on the magnet wire to permit the fusing of the wire to the pin.
As a result of the notched construction of the terminal pin, the electrodes are prevented from engagement with the magnet wire during the fusing process. This prevents the application of a compressive force to the magnet wire so as to enhance the bond between the magnet wire and the terminal pin. This inability to compress the magnet wire during the fusing process can result in poor quality bonds which may ultimately adversely effect the performance of the electronic assembly.
There is also known from Abe, U.S. Pat. No. 4,835,356, a three electrode spot welding apparatus designed for fusing magnet wire. The apparatus includes three electrodes, the first two of which are opposed to one another whereby a workpiece, including, for example, a magnet wire and a terminal pin to be welded together, is subjected to electric resistance welding in such a manner that the workpiece is held between the first and second electrodes. The third electrode is interposed between the first electrode and the workpiece so that one distal end face of the third electrode abuts against a workpiece engaging end of the first electrode and that the other distal end face of the third electrode is in contact with the workpiece. The proximal end of the first electrode is connected to a first terminal of a power source and the respective proximal ends of the second and third electrodes are connected to a second terminal.
The power source supplies a current to heat the third electrode and remove the plastic electrical insulation by vaporization by closing a circuit having a path through the first terminal, the first electrode, the third electrode and the second terminal during a predetermined period. After removal of the electrical insulation, the power supply supplies a current to weld the magnet wire to the workpiece by closing a circuit having a path through the first terminal, the first electrode, the third electrode, the workpiece, the second electrode and the second terminal. Although the disclosed fusing apparatus is useful in fusing magnet wire, it is not suitable for fusing magnet wire over a large area, e.g., circumferentially about a terminal, to achieve greater bond strength. Thus, there have been substantial unmet needs for still further improvements in a fusing apparatus and method for magnet wire and terminal pins.